Shaft seals are known devices to prevent fluid leakage along a drive shaft of a pump, motor or other fluid transfer device. One known type of shaft seal is a viscous seal. The viscous seal acts like a screw pump, forcing any leaking fluid back into the housing of the fluid transfer device. In a conventional design, the viscous seal is attached to the housing and the drive shaft rotates within a bore in the housing that is sealed by the viscous seal. The viscous seal, if working properly, will not be lubricated by the leaking fluid over its entire length since the leakage will be stopped before reaching the outboard end of the seal. Thus, the drive shaft and the viscous seal will be dry and free from lubrication over a portion thereof. For this reason, prior art viscous seals are designed not to contact the drive shaft to prevent damage to the seal or galling of the drive shaft. The effectiveness of the seal is directly proportional to the radial gap between the seal and the drive shaft. The seal can be made more effective by reducing the gap.
Prior art viscous seals have been effective for preventing leakage of relatively viscous fluids having a viscosity of about 10,000 centipoise or higher. The effectiveness of viscous seals, however, decreases as the viscosity of the leakage fluid decreases. At relatively low viscosities on the order of about 100 centipoise, other means are needed to increase the effectiveness of the viscous seal. As above noted, viscous seal performance can be improved by decreasing the clearance between the drive shaft and the seal, but there are practical limits to maintaining the alignment between the drive shaft and seal in order to prevent contact between the drive shaft and seal. Another technique is to increase the viscosity of the leakage fluid at the seal by cooling the fluid, either actively or passively. The known cooling techniques, however, may not always be suitable for a given application or can introduce undesired additional cost and/or maintenance.